Fly-by-wire technology has provided a major advance in aircraft flight control systems.
Mechanical flight control systems had been prevalent where the flight control surfaces on an aircraft were controlled using a system of cables, linkages and mechanical controls as the primary and usually only control paths. Only limited, basic mechanical failure mode contingency was possible in such systems.
With the advent of electronics and computer-aided flight controls, mechanical systems were given enhanced controllability through augmentation with, e.g., electrical drive assist and controls.
Such improvements allowed for the development of advanced control laws that, among other things, improve and increase aircraft controllability and performance. These control laws can be complex and their implementation in a mechanical control system, even those with electronic assistance, is difficult and often results in significant increase in aircraft weight.
As the next step in relevant technological evolution, mechanical linkages (rods, cables, arms, etc) were replaced by motors, actuators and drives with associated wiring as so-called “fly-by-wire” technologies were developed.
A simplified diagram of a known fly-by-wire (FBW) system is shown in FIG. 1. In such a fly-by-wire system, there is no direct mechanical coupling between the pilot controls 10 and a flight control surface 20. Known fly-by-wire systems include for example, a set of pilot controls 10 which provide electrical signals to aircraft electronics 24, where they are combined with other airplane data to produce flight control surface commands. Instead of mechanical linkages and their attendant displacement/translation, the commands are transmitted across wires 38 to electrically control the movement of actuator/motors 26 that move the flight control surfaces 20.
For purposes of safety, fly-by-wire systems typically include redundant components so that if one component of the system fails, the aircraft can still be safely controlled. In such prior art systems, it is known to provide redundancy on an axis-by-axis basis. For example as shown in FIG. 2, some prior art fly-by-wire architectures have separate systems that control the movement of the aircraft in each of the roll, pitch and yaw axes.
In these prior art systems, each axis control system typically included a dedicated primary flight computer/processor and a back-up flight computer/processor that control movement of the aircraft in a particular axis. If the primary flight computer/processor that controls one axis were also to fail, the back-up computer/processor would engage to control that axis of the aircraft. Similarly, the other axis systems would each include a primary and back-up flight computer/processor. However, if the back-up computer/processor in a particular axis channel were to fail, the computers/processors in the other axis channels could not function to fly the aircraft in the failed axis.
As an improvement to such systems, redundant multi-channel, multi-axes, fly-by-wire control systems such as shown in FIG. 3 were introduced, in which the fly-by-wire system is divided into a series of parallel independent control channels, each channel capable of flying the aircraft. Each control channel is substantially isolated from the other control channels such that the channels operate in parallel and provide redundancy where in the event of a malfunction in or of one channel, the continued operation of the remaining channels is not affected and multi-axis controllability is maintained.
With reference to FIGS. 3 and 4, one such prior art fly-by-wire system includes three isolated identical control channels—a left flight control channel 60, a center flight control channel 80 and a right flight control channel 90 each channel having its own dedicated primary flight computer 64, 84 or 94 and actuator control electronics unit (ACE) 62, 82 or 92, the ACE controlling a set of associated flight control surfaces 66. Each flight control channel is fully redundant and can control all axes of the aircraft.
As shown in FIG. 4, pilot/co-pilot control inputs (via respective transducer signals) are provided to each of ACEs 62, 82, and 92 which then make available the transducer signal over its designated one of three data buses 40, 42, and 44. The primary flight computers 64, 84, and 94 each receives transducer signals from each of the three data buses 40, 42, and 44. Each of the primary flight computers (left 64, center 84 and right 94) selects one of the transducer signals received from one of the three ACEs to use in generating a set of flight surface commands, which particular transducer signal is selected by the flight computers is based on predetermined selection rules. After selecting a transducer signal from one of the three ACEs, left primary flight computer 64 generates a set of proposed flight surface commands which are then transmitted over the data bus 40, while the center and right primary flight computers 84 and 94 transmit their sets of proposed flight surface commands on the center and right data buses 42, and 44, respectively. The left primary flight computer 64 compares the set of proposed flight surface commands it has generated with the proposed flight surface commands generated by each of the other primary flight computers 84 and 94. In a mid-value select block 379a, the left primary flight computer 64 selects the middle value of each of the flight surface commands. The left primary flight computer 64 then transmits the selected middle values of the flight surface commands over the left data bus 40, while the center and right primary flight computers transmit their selected middle value flight surface commands on the center and right data buses 42 and 44. The ACEs 62, 82 and 92 receive the respective sets of flight surface commands generated by each of the primary flight computers 64, 84, and 94. The left ACE 62 selects one of the sets of flight surface commands generated by one of the three primary flight computers in accordance with the signal select function of an input signal management block where the ACE selects the commands from the flight control computer in its own channel. Once the ACE 62 has selected a set of flight surface commands, it applies the selected set of flight surface commands to a servo loop, which controls an actuator 558a that moves the appropriate aircraft control surface. The operation of the center primary flight computer 84 and the right primary flight computer 94 are the same as that of the left primary flight computer 64 described.
In this fashion, this prior art fly-by-wire control system demonstrates a plurality of independent and isolated flight control channels each channel including an actuator controller electronics unit (ACE) that receives transducer signals that are indicative of the position of the pilot controls and a primary flight computer that is coupled to that ACE and which generates flight surface commands based on the transducer signals received from the ACE. The ACE in each isolated flight control channel receives the flight surface commands from the corresponding primary flight computer and sends the commands to a plurality of servo loops that control the movement of a set of flight control surfaces on the aircraft.
In this known system, each of the plurality of flight control channels is an independent, isolated, control channel, each of the plurality of flight control channels is of identical configuration, and all of the plurality of channels operate in parallel, with redundancy incorporated through the parallel operation.
The set of flight control surfaces controlled by each isolated flight control channel is selected so that operation of a single isolated flight control channel is sufficient to fly the aircraft in the event that the remaining isolated flight control channels fail, further characterizing the independent, isolated and redundant nature and aforementioned structure of this known flight control channel.
The foregoing is believed to describe the prior art systems as set forth, for example, in U.S. Pat. No. 5,493,497.
Such parallel structure requires that all independent channels are active in normal mode function and operate in a synchronized (e.g., real-time) fashion. Failure mode monitoring requires flight control commands to be constantly compared between the simultaneously operating redundant control channels. In the failure operation of these types of systems, if one primary flight computer fails, the remaining ones continue to operate all channels under a failure management scheme and control the aircraft.
While the above prior art fly-by-wire system provides redundant control channels such that the aircraft can be controlled safely by one channel in case of failures of other channels, one drawback of the prior art is that a generic fault at the ACE level (e.g., batch failure—manufacturing defects) could affect all ACEs, which could degrade the controllability of all surfaces. Another drawback is the need for duplicative components to achieve functional operation of such prior art redundant system.